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I already posted a lot here about the reliability of the Thielert 125 engines. A new episode in this already long story took place on the 12th of March, when the FAA issued an emergency airworthiness directive requesting: “Before the next flight, install a new high-pressure fuel line and a high-pressure fuel line
bracket”.

This does not concerns directly the high pressure pump, but the fuel line between the high-pressure pump and the common rail. This AD results from reports of in-flight engine shutdowns, as consequences of cracks in high pressure lines resulting from excessive vibrations.

The problem addressed by this AD is certainly serious. As a fan of modern engines, I heard a lot of negative comments on them, and this will certainly create a new wave of criticism.

However I find interesting that this problem is in no way related to software, electronics, FADEC or engine management… but to a fuel line. Even if classical engines don’t have such high-pressure lines, it’s nothing else than a piece of tube. No high-tech or complex thing…

Links:
AOPA information

FAA AD on AOPA server

High-pressure pump - weak point of Thielert engines ?

Side-slips in C172-TDI - High Pressure Pump issue

Thielert engines have been around for quite a while now - my personal flight time on Thielert equipped planes is close to 68 hours over two years. As far as I’m concerned these engine perform well. The only negative feed-back I can think of is about the high pressure pumps the engine uses to bring Jet-A1 fuel to the temperature and pressure conditions which make it explosive. The fuel is injected under a pressure of 1350 bars and at a temperature of 120 °C !

The planes I flew - a DA40 TDI and a C172 retrofit - both had some troubles related to the high-pressure pumps, which make me think that the pump could be the weak point of the Thielert engine, even if the troubles were of different nature.

No fuel starvation allowed
In a previous post I mentioned that the club I used to rent the Cessna from decided to forbid us to fly side-slips. This is decision was an extension of the AFM which mentions that side-slips with low fuel levels are forbidden. The reason is that the high pressure pump can be damaged after a fuel starvation of 15 seconds ! A prolongated side-slip could lead to such a condition. Any engine would probably stop running under such circumstances, but the pumps (electrical and mechanical) would not be damaged !

No water allowed
A DA40 also had its pump replaced because, according to Diamond, it had been damaged because too much water went through it. The hypothesis is that because of massive condensation and insufficient pre-flight draining left water in the tanks. It is not clear to me yet how water instead of fuel can be so bad for the pump, but I’m not an engine specialist.

Starting problems
One of the operational advantages of FADEC controlled engines is that they always start easily and at first attempt. No three hands procedure needed, nor variants for warm or flooded engine. Just press start or turn the key, and the engine fires-up… says the theory.

One of the planes I rented a lot was just unable to start when the engine was warm. A minimum cooling time was required before any successful restart. It went to the maintenance workshop several times before Thielert accepted to replace the full engine (or pump, I’m not sure now…). Measurements (the good side of FADEC controlling everything) shown that the pump was not able to reach the 300 bars needed during the starting phase.

Youth problems
None of these issues cause any engine failure in flight nor any incidents. Unexpected maintenance is always exasperating, but seldom dangerous. These problems are certainly youth problems that Thielert will cure rapidly. Don’t forget that the last “evolution” before diesel engines was injected engines ! And as far as I’m concerned, Thielert engines perform well.